Traditionally, a light emitting diode (LED) is fabricated into a parallelepiped shape. As there is a great difference between the refractive indexes of air/packaging material and a common LED, light emitted by LED is apt to be totally reflected in an LED-air or LED-packaging material interface. Therefore, light generated by LED reaching an interface by an angle greater than the total reflection critical angle will be totally reflected back to the interior of the LED chip. Besides, the parallel faces of a parallelepiped decrease the probability that light leaves an LED chip from an interface. Thus, photons can only be totally reflected inside the LED chip until they are completely absorbed and converted into heat. Therefore, LED usually has insufficient light efficiency.
Conventionally, GaN (Gallium Nitride) LED is grown on an insulating aluminum oxide substrate. An LED epitaxial layer should be etched until the n-type semiconductor is reached before the N-type electrode is formed. As the bonding energy of a GaN LED epitaxial layer is very high, it is hard to etch with a wet etching method. Therefore, a dry etching method is usually used to etch GaN LED. In the current dry etching technology, the effect of physical etching exceeds that of chemical etching; consequently, dry etching often forms a parallelepiped shape.
Changing LED shape is an effective approach to improve LED light efficiency. U.S. Pat. No. 6,229,160 disclosed a “Truncated Inverted Pyramid (TIP)” LED, wherein four faces of an AlGaInP/GaP LED chip are mechanically fabricated to be no more parallel to each other. Thus, light can be effectively extracted from LED. Thereby, the external quantum efficiency thereof is greatly increased to 55%, and the light efficiency thereof can reach as high as 1001 m/w. The TIP LED is the first LED achieving the abovementioned standard. However, the technology of TIP LED can only apply to red light AlGaInP/GaP LED chips. A GaN LED is epitaxially grown on a sapphire substrate, which is very hard to mechanically fabricate. Therefore, the TIP LED technology cannot apply to GaN LED.
U.S. Pat. No. 6,570,190 disclosed a “LED having angled sides for increased side light extraction”, whereby the external quantum efficiency thereof is greatly promoted. As the bonding energy of a GaN LED is very high, it is hard to obtain angles having special crystallographic directions via etching. If a pure chemical etching process is used, it cannot obtain any inclined faces even.